1. Field of the Invention
The present invention relates to an image forming method and apparatus, and particularly to an image forming method and apparatus in which efficiency of image formation is improved.
2. Description of the Related Art
In an image forming apparatus using an intermediate transfer member, such as a color copying machine, a single color image is developed several times and transfer is carried out while colors are superimposed on the intermediate transfer member at the same time. When images of all colors are transferred onto the intermediate transfer member and a final color image is formed, the image is transferred onto a transfer sheet, the image on the transfer sheet is fixed by a fixing device, and the transfer sheet is discharged to the outside of the apparatus.
In this case, each time an image is developed, a read operation of an original document is performed, and read data is converted into single color data for print. For example, in the case where printing is performed with four colors of yellow, magenta, cyan and black in an image forming apparatus having a structure as shown in FIG. 7, when a copy start operation is made by an operation portion 105, a control portion 104 controls a read portion 101 and the like to start an operation. First, image data of yellow is outputted from an image processing portion 102 by a first read operation of the read portion 101, and an image of yellow is formed on a photoreceptor in a print portion 103. With a slight delay, the formed image is transferred from the photoreceptor onto the intermediate transfer member.
When the intermediate transfer member makes one rotation and again returns to the original position, the read portion 101 performs a second read operation, and by this, image data of magenta is outputted from the image processing portion 102. In the print portion 103, an image of magenta is formed on the photoreceptor, and with a slight delay, the formed image of magenta is transferred from the photoreceptor onto the intermediate transfer member.
When formed images of cyan and black are transferred onto the intermediate transfer member in the same manner, the images on the intermediate transfer member are transferred onto a transfer sheet, and the transfer sheet is subjected to image fixation by a fixing device and is discharged to the outside of the apparatus.
In such an image forming apparatus, each time the intermediate transfer member makes one rotation, only image formation for one color of one transfer sheet can be performed. On this account, even in the case where image formation to a small transfer sheet is performed, since image formation of a next color can not be performed until the intermediate transfer member makes one rotation and returns to the original position, the efficiency is poor. For example, in the case where image formation is performed to a transfer sheet of A4 short-edge feed size in an image forming apparatus which can make printing up to a transfer sheet of A3 long-edge feed size at the maximum, an image is formed on only a front half of the intermediate transfer member, and a rear half of the intermediate transfer member is not used. The same applies to the case where an image is continuously formed from the same original document. Thus, the number of continuous output sheets per unit time becomes the same irrespective of the size of a transfer sheet.
Then, an image forming method with a two-sheet mode has been proposed in which in the case where an image is continuously formed on a transfer sheet of a size equal to or smaller than half of the maximum transfer sheet size, two images are continuously formed on an intermediate transfer member, and the images are finally continuously transferred onto the transfer sheet. This mode is a method in which two read operations are performed while the intermediate transfer member makes one rotation, and the same image of the same color is continuously developed on the intermediate transfer member. For example, in the case where an image is formed on a transfer sheet of A4 short-edge feed size in an image forming apparatus which can form an image on a transfer sheet of A3 long-edge feed size at the maximum, an image is formed on a front half of an intermediate transfer member, and further, the same image is formed also on a rear half. This operation is performed for all colors, and finally, the formed images are continuously transferred onto two transfer sheets. By this, the number of continuous output sheets per unit time can be made approximately double.
However, in this method, since two read operations become necessary for one rotation of the intermediate transfer member, it is necessary that a read movable portion returns to a read start position in a time in which the intermediate transfer member moves through a short blank portion between formed images continuous on the intermediate transfer member. Further, in an image forming apparatus having a function of enlargement/reduction copying, as a reduction rate becomes small, a read range of an original document becomes wide, and a time required for the read movable portion to return to the read start position becomes long.
In order to cope with this problem, it is necessary that the returning speed of the read movable portion is made high, or the intermediate transfer member is enlarged to extend an interval between formed images. However, if the returning speed of the read movable portion is made high, consumption power of a motor is increased, or the motor itself is enlarged, that is, there is such a problem as an increase in cost. Besides, if the size of the intermediate transfer member is made large, there is such a problem that the number of continuous copies per unit time is decreased to the contrary, and the size of the copying machine itself becomes large.
As a technique for solving such problems, there is an image forming apparatus disclosed in Japanese Unexamined Patent Publication No. Hei. 3-154072. In this image forming apparatus, in the case where a reduction rate becomes equal to or smaller than a constant range, irrespective of a size of a transfer sheet, only one image is always formed on an intermediate transfer member. However, also in this case, eventually, the number of continuous output sheets per unit time below a certain reduction rate becomes small.
Besides, as a method of solving these problems, it is possible to conceive such a method that all read data is once stored in a memory, and with respect to the output of a second sheet and the following, the read data is read out from the memory to make printing. If such a way is adopted, a read movable portion has only to read an image once.
For example, in an image forming apparatus having a structure as shown in FIG. 8, image data read by a read portion 201 is stored in memory portions 206a, 206b and 206c. The memory portions 206a, 206b and 206c respectively store images of red, green and blue, and an image processing portion 202 reads out this and converts it into yellow, magenta, cyan and black, and output is made from a print portion 203.
Besides, in an image forming apparatus having a structure as shown in FIG. 9, image data converted into yellow, magenta, cyan and black, not three colors of red, green and blue, are stored in memory portions 306a, 306b, 306c and 306d, respectively. In this case, since image data of three colors of red, green and blue are not stored, when a read portion 301 reads an original document, it is necessary to perform conversion into yellow, magenta, cyan and black at the same time, and four image processing portions 302a, 302b, 302c and 302d are used. Then, switching of a switch 308 is made, and image data stored in the memory portions 306a, 306b, 306c and 306d are sequentially outputted to a print portion 303 and printing is made.
Like this, in the case where read data is once stored in a memory, a large amount of memory becomes necessary.
As described above, in the conventional image forming apparatus, in the case where an attempt is made such that plural images are formed on the intermediate transfer member and continuous output is made, although there has been conceived the method in which the output of the motor for driving the read movable portion is increased and the size of the intermediate transfer member is enlarged, or the method in which read data is stored in a large capacity memory, either case has caused an increase in cost. Incidentally, the same problem occurs also in a single color image forming apparatus in which plural images are formed on an intermediate transfer member, or in a single color image forming apparatus in which plural images are formed on a photoreceptor without using an intermediate transfer member and the images are directly transferred from the photoreceptor onto a transfer sheet.
The present invention has been made in view of the above circumstances and provides an image forming method and apparatus in which plural identical images are formed on an intermediate transfer member or the like without using a large capacity memory and output efficiency can be improved.
According to an aspect of the present invention, an image forming method for reading an original document and forming an image read from the original document on a sheet has the steps of storing image data read from the original document and forming a first latent image based on the stored image data, reading out the stored image data and forming a second latent image based on the read out image data, developing the first latent image and the second latent image to form a first developed image and a second developed image, respectively, and transferring the first developed image and the second developed image onto a first sheet and a second sheet, respectively.
According to another aspect of the present invention, an image forming apparatus which reads an image from an original document and forms the image on a sheet has a reading unit that reads the original document to obtain image data, a storage unit that stores the image data obtained by the reading unit, a latent image forming unit that forms a latent image based on the image data stored in the storage unit, and a transfer unit that develops the latent image formed by the latent image forming unit and transfers it onto a sheet. The latent image is constituted by a first latent image and a second latent image. The transfer unit develops the first latent image and the second latent image and transfers them onto a first sheet and a second sheet, respectively.